16.4 Momentum and Radiation Pressure

Electromagnetic waves carry momentum, so they exert pressure when they strike a surface. Reflective surfaces experience more radiation pressure than absorbing surfaces because the wave's momentum changes more. That small force can shape comet tails and propel spacecraft using light sails.

Radiation pressure shows how light can produce measurable physical effects. It connects electromagnetic waves to real applications in space exploration, where even a small continuous force can matter over long distances.

Momentum and Radiation Pressure

Radiation pressure on surfaces

• Radiation pressure exerted by electromagnetic waves on a surface when waves transfer momentum upon absorption or reflection
• Absorbing surfaces experience radiation pressure $P = \frac{I}{c}$
  • $P$ radiation pressure
  • $I$ intensity of electromagnetic wave (power per unit area)
  • $c$ speed of light (3 × 10^8 m/s)
• Reflecting surfaces experience doubled radiation pressure $P = \frac{2I}{c}$
  • Change in momentum twice as large compared to absorbing surfaces
• Higher intensity electromagnetic waves lead to greater radiation pressure (sunlight vs laser)
• Surface properties affect pressure
  • Absorbing surfaces experience less pressure (black paint)
  • Reflecting surfaces experience more pressure (mirror)

Radiation effects on comets

• Comets affected by radiation pressure from the Sun
  • Increasing radiation pressure as comet approaches the Sun
• Comet's tail points away from the Sun due to radiation pressure
  • Tail consists of dust and gas particles pushed away by Sun's radiation
  • Always points away regardless of comet's direction of motion
• Radiation pressure can alter comet's orbit over time
  • Force exerted causes small deviations in trajectory (Halley's Comet)
  • Deviations accumulate, leading to changes in orbital path (period and shape)

Applications in space exploration

• Light sail propulsion method relies on radiation pressure
  • Large, lightweight sail deployed to catch radiation from Sun or powerful lasers
  • Radiation pressure exerts force on sail, propelling spacecraft forward
• Advantages of light sail propulsion:
  • No onboard propellant required, reducing spacecraft mass and cost
  • High speeds achievable over long distances
    • Suitable for interplanetary or interstellar travel (Breakthrough Starshot)
  • Sun's radiation or ground-based lasers used as power source
• Challenges of light sail propulsion:
  • Requires large, thin, durable sail materials
    • Maximize radiation pressure, minimize mass (mylar, graphene)
  • Navigation and control difficult due to limited maneuverability
  • Dependence on external radiation sources limits trajectory flexibility
• Potential applications:
  • Interplanetary missions within solar system (Solar Cruiser)
  • Interstellar probes for exploring nearby star systems (Project Dragonfly)
  • Deorbiting space debris using radiation pressure to remove defunct satellites (CubeSail)

Fundamental concepts

• Photons: particles of light that carry momentum and energy
• Conservation of momentum: principle applied when photons interact with surfaces
• Force: exerted on surfaces due to change in photon momentum
• Pressure: force per unit area, describing the effect of radiation on surfaces
• Energy: transferred from electromagnetic waves to surfaces during interactions
• Maxwell's equations: describe the behavior of electromagnetic waves, including their momentum-carrying properties